The field of the invention relates to gas turbines, and, in particular but not limited, to liquid fuel injection systems for industrial gas turbines.
Industrial gas turbines are often capable of alternatively running on liquid and gaseous fuels, e.g., natural gas. These gas turbines have fuel supply systems for both liquid and gas fuels. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off. Similarly, when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off. Fuel transitions occur during the operation of the gas turbine as the fuel supply is switched from liquid fuel to gaseous fuel, and vice versa.
Gas turbines that burn both liquid and gaseous fuel require a liquid fuel purge system to clear the fuel nozzles in the combustors of liquid fuel. The liquid fuel supply system is generally turned off when a gas turbine operates on gaseous fuel. When the liquid fuel system is turned off, the purge system operates to flush out any remaining liquid fuel from the nozzles of the combustor and provide continuous cooling airflow to the nozzles.
FIG. 1, shows schematically a gas turbine 100 having liquid fuel system 102 and a liquid fuel purge system 104. The gas turbine is also capable of running on a gas, such as natural gas, and includes a gaseous fuel system 106. Other major components of the gas turbine include a main compressor 108, a combustor 110, a turbine 112 and a controller 114. The power output of the gas turbine is a rotating turbine shaft 116, which may be coupled to a generator that produces electric power.
In the exemplary industrial gas turbine shown, the combustor may be an annular array of combustion chambers, i.e., cans 118, each of which has a liquid fuel nozzle 120 and a gas fuel nozzle 122. The combustor may alternatively be an annular chamber. Combustion is initiated within the combustion cans at points slightly downstream of the nozzles. Air from the compressor 108 flows around and through the combustion cans to provide oxygen for combustion. Moreover, water injection nozzles 111 are arranged within the combustor 110 to add energy to the hot combustion gases and to cool the combustion cans 118.
FIG. 2 shows a conventional liquid fuel purge system 104 for a liquid fuel system. When the gas turbine 100 operates on natural gas (or other gaseous fuel), the liquid fuel purge system 104 blows compressed air through the nozzles 120 of the liquid fuel 102 system to purge liquid fuel and provide a flow of continuous cooling air to the liquid fuel nozzles 120.
The air used to purge the liquid fuel system is supplied from a dedicated motor (M) controlled purge compressor 128. The purge compressor boosts the compression of air received from the main compressor 108 via compressor discharge 202. A compressor air pre-cooler 164, separator 166 and filter 168 arrangement is used to treat the compressor air before it is boosted by the purge compressor 128. A tuning orifice 132 meters the flow of purge. The purge air from the purge compressor is routed through piping 130, a strainer 162, a Tee 137 that splits the purge airflow between the liquid fuel purge system 104 and a water purge system 126. A liquid fuel purge multiport valve 138 routes the boosted pressure purge air to each of the liquid fuel nozzles 120. The multiport valve is controlled by a solenoid 139 that is operated by the controller 114. At each combustion chamber, end cover check valves 147 prevent liquid fuel from back flowing into the purge system. In addition, the purge compressor provides air through another tuning orifice 133 to an atomizing air manifold 134 and to the atomizing air ports of the liquid fuel nozzles 120.
The liquid fuel check valves 165, at least one for each combustion chamber, isolate the liquid fuel supply 172 during purge operations and prevent purge air from back-flowing into the liquid fuel system. By preventing purge air from entering the liquid fuel system, the check valves avoid air-fuel interfaces with the fuel supply.
When the liquid fuel purge system 104 is initiated, a solenoid controlled soft purge valve 140 is open simultaneously with the multiport valve 138 by a common solenoid valve 139. The soft purge valve 140 opening rate is mechanically controlled by a metering valve in an actuation line (not shown). The soft purge valve opens over a relatively long duration of time to minimize load transients resulting from the burning of residual liquid fuel blown out into the combustor from the purge system piping 142 and the liquid fuel nozzles. The soft purge valve 140 is a low flow rate valve, to reduce the boosted pressure purge air flowing from the purge compressors. After the soft purge valve has been opened a predetermined period of time, a high flow purge valve 144 is opened to allow the boosted purge air to flow at the proper system pressure ratio. The high flow purge valve may be a two-way ball valve 144.
The above-described piping, valves, purge compressor and other components of the liquid fuel purge system are complicated and cumbersome. The system requires controlled opening of several valves, multiport valves, metering tuning orifices, check valves, all of which require maintenance and are possible failure points. If the purge system fails, component failures will likely go undetected until turbine operation is ultimately affected, at which time the turbine must be taken off-line and serviced. To avoid having to take a gas turbine off-line due to a purge system failure, the conventional wisdom has been to add more purge system components and to add a backup system to the main purge system.
For example, if the purge compressor 128 fails, then air for the purge systems is supplied from an atomizing air compressor 150 and cooled in a purge air cooler 152. When the atomizing air compressor operates to provide air for the purge systems, then motor (M) operated valves 154, 156, are closed to reduce flow and pressure, and air is routed through the purge cooler at the appropriate pressure and temperature. In addition, motor operated valve 158 is opened to provide a surge protection feedback loop. The operation of these valves 154, 156 and 158 controls the air flow to and from the atomizing air compressor 150.
Purge air from the atomizing air or purge air compressor passes through a strainer 162 to remove contaminants from the purge air and protect the contaminant sensitive components from start up and commissioning debris. The purge cooler 152 is in addition to the precooler 164, separator 166 and filter 168 used to cool air from the main compressor 108.
The previously-described conventional liquid fuel purge system has long suffered from several disadvantages and is prone to failure. To overcome the disadvantages of prior systems, the conventional wisdom has been to regularly redesign the components of the purge system, especially those components, e.g., check valves 147 and multiport valve 138, that are prone to failure due to contaminants in the purge air.
Check valves do not provide optimal isolation of the purge and fuel systems. When they fail in an open position, purge check valves allow fuel to leak into the purge system. When purge check valves fail closed, purge air does not reach the fuel nozzles, and nozzle coking and melting can occur. When a fuel check valve fails in a closed position, it prevents fuel flow to a nozzle and can create pressure head differences in the fuel system between the combustors. Failure of the fuel check valves (either open or closed) may also lead to ignition and cross-fire failures and damage to the fuel system upstream of the fuel check valves. When they fail in an open position, fuel check valves may allow purge air to bubble into the fuel system. Check valve failures lead to serious combustion problems and may force the gas turbine to be shut down for repair.
Liquid fuel check valves do not provide bubble tight isolation against purge air pressure which results in a liquid fuel/air interface. This fuel/air interface results in xe2x80x9ccokingxe2x80x9d of the liquid fuel and, thus, fouling of the liquid fuel check valves and fuel nozzles. Fouling, and in some cases plugging, of the fuel nozzles disrupts fuel flow and eventually results in high temperature spreads at which point the turbine can no longer operate on liquid fuel. The leaking check valves also allow air entrapment and back-flow of purge air into the liquid fuel system. These problems can result in false starts and can prevent gas to liquid fuel transfers during gas turbine operation. In addition, utilizing two separate components may result in improper isolation and cause the purge system to be partially back filled with liquid fuel. If the liquid fuel seeps into the purge system, the fuel may experience coking that results in blockage of the fuel nozzles, a reduction in the required purge flow and thus premature failure of the liquid fuel nozzles due to lack of purge cooling. Fuel in the purge system may also cause ignition and cross-fire failures resulting in combustion spreads between the cans and ultimately tripping of the gas turbine unit.
Moreover, functioning fuel check valves may require substantial fuel pressure to open and allow fuel to pass. The pressure required to operate the liquid fuel check valve increases the load on the fuel pump. The added load on the pump may require larger fuel pumps and/or purge compressors than would otherwise be needed.
The conventional purge control method has been to utilize a series of tuning orifices to balance the purge air and to set the appropriate pressure ratios for acceptable combustion dynamics. These tuning orifices have had to be individually sized to adjust the pressure ratios of the purge air. Furthermore, the conventional purge systems require subsystems, such as a soft purge valve 140 with tuned needle valves, for initial application of purge air to the nozzles of the liquid fuel system. The soft purge valve was added to minimize transient load spikes during fuel transfers when the purge systems are started.
With the addition of purge compressors, backup systems for the purge compressors, tuning orifices, strainers, subsystems and other new components, instrumentation had to be added to protect the new components against contamination. These fixes to the purge systems were marginally acceptable. The conventional purge air systems, with all of their fixes and new components, were complex, delicate and not adequately reliable.
Applicants designed a novel fuel purge system for a gas turbine that includes a three-way liquid fuel purge valve. The three-way valve simplifies the purge system by replacing the prior two-way purge valves, check valves, poppet multiport valves, Tee junctions and other components of prior liquid fuel purge systems. At least one three-way valve couples both the liquid fuel supply and the purge air system to an end cover of each combustion chamber can. The valve switches the flow of purge air to the fuel nozzles to liquid fuel flow, and vice versa. The three-way valve retains less liquid fuel volume, e.g., 22% less, than does the equivalent combination of a two-way liquid fuel purge end-cover isolation valve (or a purge check valve), liquid fuel check valve and Tee-assembly. The lower fuel volume in the valve reduces the volume of liquid fuel to be purged and thereby reduces the transition magnitude when switching from liquid to gaseous fuel.
In addition, the three-way valve prevents back-flow or purge air into the liquid fuel system, and vice versa. Back-flow was previously prevented by liquid fuel check valves that are prone to coking (a condition where internal air passages that are exposed to fuel become varnished with fuel residue) and contamination. Similarly, the prior poppet-type multiport valve, purge isolation valves and fuel check valves were adversely affected by contaminants in the purge air. The three-way valves also eliminate (or at least markedly reduce) the potential of liquid fuel back-flow into the purge air manifold during liquid fuel operation, and especially during fuel transitions.
The three-way valve system has passive and active modes. During the active mode, the valve is controlled by external signals, such as instrument air pressure applied by the gas turbine controller. In passive mode, the valve is controlled by the pressure of the liquid fuel. The passive mode is used to switch the valve between purge air flow and purge liquid fuel flow. The active mode is applied to hold the valve in a liquid fuel ON flow setting during high fuel-flow conditions. The active mode is not used to switch the valve from fuel flow to purge air, or vice versa. The valve is biased to purge air flow, if there is insufficient fuel pressure present to operate the valve.
The advantages provided by passive/active modes include providing uniform back pressure to the liquid fuel system to balance pressure head differences between combustor cans, minimizing the risk that hot fuel nozzles lose both cooling air and liquid fuel flows simultaneously, reducing the pressure demand on liquid fuel pumps, providing fail-safe valve operation, minimization of purge system components and improved reliability.
In the present invention, the three-way valves (operating in the passive mode) automatically switch to pass fuel to the nozzles when the fuel pressure increases. The fuel pressure increase is the actuating force that switches the valve from applying purge air to applying liquid fuel flow to the fuel nozzles. Pressure head differences (and the corresponding pressure induced stresses) in the liquid fuel system are minimized by eliminating the potential that a fuel check valve fails open or closed. Accordingly, there is minimal risk that excessive pressure head differences between the combustors will occur in the liquid fuel system because of a spool type three-way valve that replaces the failure prone poppet type check valves.
The need for large, high pressure liquid fuel pumps is reduced because the check valves are no longer needed that had applied substantial back pressure to fuel pumps. In the past, high pressure check valves were actuated by high fuel pressure and, thus, increased the load on the fuel pump. The size of a fuel pump is dependent on the required fuel pressure, especially during high fuel-flow conditions. To remain open to fuel flow, check valves applied substantial back pressure to fuel pumps, including during high fuel flow conditions. The fuel pressure needed to operate the three-way valve of the present invention is less than the pressure required to open the prior high pressure check valves. Moreover, during high liquid fuel-flow conditions, the three-way valve of the present invention is in active switch mode such that instrument air is applied to the valve actuator. High liquid fuel pressure is not needed to operate the valve when in active mode. Since the fuel pump is not required to operate the valve during high fuel flow mode, smaller (and hence more economical) fuel pumps may be used. These smaller fuel pumps are sufficient to provide the fuel pressure needed to operate the three-way valve during passive mode.
The purge system of the present invention is simple, robust, reliable and cost effective. This system provides a continuous and reliable flow of purge air to flush the nozzles for liquid fuel and water injection free of liquids, and to cool the nozzles. In addition, the three-way valve of the purge system prevents back-flow of hot combustion products into the liquid fuel system. Furthermore, when the fuel system is on, it is isolated from the purge system by the three-way valve to prevent accumulation of fuel in the purge system.
Further, advantages provided by the purge system of the present invention include enhanced reliability in the operability of the liquid fuel systems for gas turbines, and improved transient attributes of purge systems during liquid fuel to gas fuel transitions. The inventive purge system provides a continuous flow of purge air to flush liquid fuel from fuel nozzles, to cool the nozzles and prevent back-flow through the nozzles and liquid fuel manifold of hot combustion products when liquid fuel is not flowing. In the present invention the purge and liquid fuel systems work together to prevent back-flow of purge air into the liquid fuel system to prevent liquid fuel xe2x80x9ccokingxe2x80x9d and air entrapment in the liquid fuel system when liquid fuel is not flowing through the fuel system. The purge system also provides isolation when liquid fuel is flowing by preventing the accumulation of liquid fuel in the purge system.
The inventive purge system with three-way valve may operate with lower pressure air from the main compressor discharge, i.e., a compressor-less purge system, and does not require a separate purge compressor to boost the pressure of the purge air while the gas turbine operates on gas fuel. The main compressor is inherently reliable, at least in the sense that the gas turbine cannot operate when the main compressor is inoperable. In addition, the atomizing air compressor is not needed as a back-up boost pressure system while the gas turbine is on gas fuel. To accommodate the lower pressure purge air, the purge air piping may have increased diameters to allow for greater purge air flow volume. In addition, the present purge system includes a purge manifold to distribute purge air to the liquid fuel nozzles. This manifold replaces the complex multiport poppet valve used on conventional purge systems.
Other novel features of the present invention include true block-and-bleed capability which provides double valve isolation with an inter-cavity vent for improved reliability, and a single point tuning control valve that allows adjustments to be easily made to the pressure ratio required for minimum combustion dynamics.